First of all, 100 lines is a small file, not a big one ;-)

If you just need to copy the file and do something to every line, this might be something similar to what you want (untested, written from memory):

#!/usr/bin/env perl
use strict;
use warnings;

open(my $ifh, '<', 'infile.txt') or die($!);
open(my $ofh, '>', 'outfile.txt') or die($!);
while((my $line = <$ifh>)) {
    chomp $line;

    # do something to $line

    print $ofh $line, "\n";
}
close($ifh);
close($ofh);
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If you really just want to copy a file, you can use File::Copy:

#!/usr/bin/env perl
use strict;
use warnings;

use File::Copy;

copy('infile.txt', 'outfile.txt') or die("Ooops, i messed up: $!");
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