substr() indexing is wrong on very big numbers

Above a certain value, the output of substr EXPR,OFFSET is the last character of the string

$ perl -E '$var="hello"; say substr($var, 3 . "0" x 18)'


$ perl -E 'say substr("hello", 18446744073709551615)'


$ perl -E 'say substr("hello", 18446744073709551616)'
o

$ perl -E '$var="hello"; say substr($var, 3 . "0" x 19)'
o

$ perl -E '$var="hello"; say substr($var, 3 . "0" x 200)'
o

$ perl -E '$var="hellA"; say substr($var, 3 . "0" x 19)'
A

$ perl -E '$var="hellE"; say substr($var, 3 . "0" x 19)'
E

And above the same value, the output of substr EXPR,OFFSET,LENGTH chop off the last character of the string

$ perl -E '$var="hello"; say substr($var, 0, 3 . "0" x 18)'
hello

$ perl -E '$var="hellA"; say substr($var, 0, 3 . "0" x 18)'
hellA

$ perl -E 'say substr("hello", 0, 18446744073709551615)'
hello

$ perl -E 'say substr("hello", 0, 18446744073709551616)'
hell

$ perl -E '$var="hello"; say substr($var, 0, 3 . "0" x 19)'
hell

$ perl -E '$var="hellA"; say substr($var, 0, 3 . "0" x 19)'
hell

$ perl -E '$var="hello"; say substr($var, 0, 3 . "0" x 100)'
hell

$ perl -E '$var="hellA"; say substr($var, 0, 3 . "0" x 100)'
hell

$ perl -V
Summary of my perl5 (revision 5 version 36 subversion 0) configuration:
   
  Platform:
    osname=linux
    osvers=5.12.15-arch1-1
    archname=x86_64-linux-thread-multi
    uname='archlinux'
    config_args='-des -Dusethreads -Duseshrplib -Doptimize=-march=x86-64 -mtune=generic -O2 -pipe -fno-plt -fexceptions         -Wp,-D_FORTIFY_SOURCE=2 -Wformat -Werror=format-security         -fstack-clash-protection -fcf-protection -flto -Dprefix=/usr -Dvendorprefix=/usr -Dprivlib=/usr/share/perl5/core_perl -Darchlib=/usr/lib/perl5/5.36/core_perl -Dsitelib=/usr/share/perl5/site_perl -Dsitearch=/usr/lib/perl5/5.36/site_perl -Dvendorlib=/usr/share/perl5/vendor_perl -Dvendorarch=/usr/lib/perl5/5.36/vendor_perl -Dscriptdir=/usr/bin/core_perl -Dsitescript=/usr/bin/site_perl -Dvendorscript=/usr/bin/vendor_perl -Dinc_version_list=none -Dman1ext=1perl -Dman3ext=3perl -Dcccdlflags='-fPIC' -Dlddlflags=-shared -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -flto -Dldflags=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -flto'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
  Compiler:
    cc='cc'
    ccflags ='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    optimize='-march=x86-64 -mtune=generic -O2 -pipe -fno-plt -fexceptions -Wp,-D_FORTIFY_SOURCE=2 -Wformat -Werror=format-security -fstack-clash-protection -fcf-protection -flto'
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
    ccversion=''
    gccversion='12.1.0'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=16
    longdblkind=3
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='cc'
    ldflags ='-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -flto -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib
    libs=-lpthread -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc -lgdbm_compat
    perllibs=-lpthread -ldl -lm -lcrypt -lutil -lc
    libc=/lib/../lib/libc.so.6
    so=so
    useshrplib=true
    libperl=libperl.so
    gnulibc_version='2.35'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E -Wl,-rpath,/usr/lib/perl5/5.36/core_perl/CORE'
    cccdlflags='-fPIC'
    lddlflags='-shared -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -flto -L/usr/local/lib -fstack-protector-strong'


Characteristics of this binary (from libperl): 
  Compile-time options:
    HAS_TIMES
    MULTIPLICITY
    PERLIO_LAYERS
    PERL_COPY_ON_WRITE
    PERL_DONT_CREATE_GVSV
    PERL_MALLOC_WRAP
    PERL_OP_PARENT
    PERL_PRESERVE_IVUV
    USE_64_BIT_ALL
    USE_64_BIT_INT
    USE_ITHREADS
    USE_LARGE_FILES
    USE_LOCALE
    USE_LOCALE_COLLATE
    USE_LOCALE_CTYPE
    USE_LOCALE_NUMERIC
    USE_LOCALE_TIME
    USE_PERLIO
    USE_PERL_ATOF
    USE_REENTRANT_API
    USE_THREAD_SAFE_LOCALE
  Built under linux
  Compiled at May 29 2022 08:49:34
  %ENV:
    PERL5LIB="/home/USER/.my_configurations/perl_modules"
  @INC:
    /home/USER/.my_configurations/perl_modules
    /usr/lib/perl5/5.36/site_perl
    /usr/share/perl5/site_perl
    /usr/lib/perl5/5.36/vendor_perl
    /usr/share/perl5/vendor_perl
    /usr/lib/perl5/5.36/core_perl
    /usr/share/perl5/core_perl

Start by reproducing the reported substr EXPR,OFFSET and substr EXPR,OFFSET,LENGTH cases on the stated Perl 5.36 configuration, then trace the substr entry point and its numeric argument handling. Identify the relevant regression-test location and verify that offsets and lengths above the 64-bit boundary preserve the expected string behavior.