$! was an unfortunate choice of variable. :-) It's a dualvar so it has different values if treated as a string or as a numerical.

open my $fh, '<', 'this does not exist';
printf "%s (%d)\n", $!, $!;

__END__
No such file or directory (2)
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It's better to just create a copy and pass that, if the subroutine doesn't copy the argument.

lodin