#!/usr/bin/perl

use strict;

my %hash;

open (DATA, "<.txt") or die "Can't open file $!\n";
my @DATA = <DATA>;
close (DATA);

foreach my $rec (@DATA) {
  chomp $rec;
  my @vars = split(/,/,$rec);
  if ($vars[0]) {
    # you can use map to use the regex
    # on each element of the array
      map  $_ =~ s/^\s+$//g, @vars;      

    print "$vars[0] $vars[1] $vars[2] $vars[3]\n";

    if( $hash{$vars[2]} ) {
      warn "key '$vars[2]' already exists in hash\n";
    } else {
      $hash{$vars[2]} = join(' ', @vars[0,1,3]);
        # or whatever you wanted
    }   # your hash to be built as
  }
}