#%hFirst stores lines loaded in from first file
#%hSecond stores lines loaded in from second file
#%hThird stores lines loaded in from third file
#OUT is a file handle to your one and only output file

while (my ($sKey, $fld1) = each(%hFirst)) {
   my $fld1 = $hFirst{$sKey};
   my $fld2 = exists $hSecond{$sKey}
      ? $hSecond{$sKey} : "NOT DEFINED";
   my $fld3 = exists $hThird{$sKey}
      ? $hThird{$sKey} : "NOT DEFINED";
   print OUT "$fld1======$fld2======$fld3\n";
}