use strict;
open my $ALUS, "Z:\\3\' UTR intron project\\Alu_bed.bed"  or die "Couldn't open alus: $!";

my %seen;
while (<$ALUS>){
    /(chr.*?)\t/ or next;
    if (! exists $seen{$1}){
      open my $fh, ">", "Z:\\3\' UTR intron project\\Alu_chroms\\$1.txt"||die "couldn't open $1 file!";
      $seen{$1} = $fh;
    }
    print {$seen{$1}} "$_";
}