package Foo;

use strict;
use warnings;
use autodie;
use lib qw( . );
use Foo qw( frobnicate );

open FH, '+<', 'data.txt';

while (<FH>)
{
    print "> $_";
}

open my $new_fh, '>', 'more_data.txt';

Foo::frobnicate($new_fh);

close $new_fh;

open FH, '<', 'data.txt';

print "\n";

while (<FH>)
{
    print ">>> $_";
}

close FH;