File example.pl:

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;
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File Foo.pm:

package Foo;

use strict;
use warnings;

our @EXPORT = qw( frobnicate );

warn "Foo.pm\n";

sub frobnicate
{
    print FH "XYZ\n\n";
}

1;
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In example.pl, the intention is to create, and then write to, a new file; but a mistake in Foo.pm causes an existing file to be written-to instead. This occurs without even a warning, because the filehandle in question is global. Giving it lexical scope would highlight the error immediately.