Hi stevieb, that's odd!

Same results here on 5.22 and 5.8.8 under darwin. It seems to be an issue with variable interpolation by Benchmark:

use strict; use warnings;
use Benchmark qw(:all);

cmpthese( 1, {
    p_read => "p_read('a.txt')",
});

sub p_read {
    my $fname = shift;
    print "$fname\n";
}

__END__
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Output:

a.txt
    ...
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