#!/usr/bin/perl -l

sub foo {
  open "/foo";
  print 0+$!,    " $!";
  print 0+$_[0], " $_[0]";
}

# not copied
mkdir "/foo";
foo $!;

print "---";

# copied with string interpolation
mkdir "/foo";
foo "$!";

print "---";

# copied with temp scalar
mkdir "/foo";
foo ${ \(my $t = $!) };

print "---";

# copied with array deref
mkdir "/foo";
foo @{[$!]};
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And the output:

2 No such file or directory
2 No such file or directory
---
2 No such file or directory
0 Permission denied
---
2 No such file or directory
13 Permission denied
---
2 No such file or directory
13 Permission denied
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