print "$^V\n";
my $x = '0123456789';
{
my $s = "$x$x";
$s =~ s/(.)$/a/;
printf "%x\n", unpack "Q", pack "P", $s;
$s =~ s/(.)$/b/;
printf "%x\n", unpack "Q", pack "P", $s;
}
{
my $s = "$x$x";
$s =~ s/.$//;
printf "%x\n", unpack "Q", pack "P", $s;
$s =~ s/.$//;
printf "%x\n", unpack "Q", pack "P", $s;
}

__END__

v5.18.4
29cdda8
29cdda8
29ce1e8
29ce1e8

v5.20.3
2b774f8
2b774f8
2b76b78
2b76f38

v5.26.3
2c9e118
2c9d7b8
2c9da88
2b4ae88