my $ERRORCOUNT=0;
my %HASH;
my %OTHERHASH;
...
foreach my $LINE (@LINES)
{
   # the sub-routine works and is returning the desired output
   my ($vlan, $mac, $int, $counter) = linesifter($LINE);

   if ($error eq 1)
   {
      ERRORCOUNT++;
   } 
   else
   {
      my @VLANS, @MACS, @INTS);
      push $HASH{$MACS[$mac]}, [$VLANS[$vlan], [$INTS[$int]];
}

# here will follow similar code from another file 

# compare $HASH[$MACS[keymatch]] to $OTHERHASH[$MACS[$keymatch]] 
# then do *magic* and line up all items that are referenced by matching MAC key in both files onto one line. Store in a file. 

sub linesifter
{
#...
#...omitted
  # various regex'es for grep'ing data
#...
return ($vlan, $mac, $int, $error);
}