UNIX for Dummies Questions & Answers

#!/usr/bin/perl

use strict;
use warnings;

open (my $in , '<', 'input.txt')|| die "Couldn't open input.txt\n\t$!";
open (my $yes, '>', 'accepted.txt')|| die "Couldn't open accepted.txt\n\t$!";
open (my $no , '>', 'rejected.txt')|| die "Couldn't open rejected.txt\n\t$!";
my @regexen=(qr'(^[0-9a-zA-Z]+$)',qr'(^[0-9]+$)',qr'(^[MF]$)');
while(<$in>){
        my $valid=1;
        my @fields = split(/\|/, $_);
        for (0..$#fields){
                if ($fields[$_]=~$regexen[$_]){
                        $fields[$_]=$1;
                }
                else {
                        $fields[$_]="";
                        $valid=0;
                }
        }
        print $yes join('|', @fields),"\n";
        if (! $valid){
                print $no $_;
        }
}
close($yes);
close($no);
close($in);
print "The valid fields in each record from input.txt are in the file accepted.txt and the invalid are in rejects.txt\n";
exit;