If using a separate function for data processing, it's generally best to keep the file reading out of it. I'd just pass a reference to the array. Heck, you can pass the array itself - Perl passes it internally as a reference anyway, and you don't lose any efficiency so long as you don't assign (copy) the contents.

GOOD:

fast(@data);
sub fast {
    my $x = $_[0];
}
[download]

BAD:

slow(@data);
sub slow {
    my @arr = @_;
    my $x = $arr[0];
}
[download]

ALSO GOOD (but not as pretty):

fast(\@data);
sub fast {
    my $arr = $_[0];
    my $x = $arr->[0];
}
[download]