You are treating @contents as a global.

I would do this like:

use strict;
use IO::File;

my @contents = open_file('myfile.txt');

sub open_file {
 my $fh = IO::File->new($_[0], 'r')
  or die "Could not open file $_[0]:$!";

 # slurp mode only if the don't want an array
 local $/ unless wantarray;

 return <$fh>;
}
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