How about this? (I tried to made it simple and readable, instead of terse. But ask if you do not understand a part, or do not know how to extend it to what you need)

#!/usr/bin/perl
use strict;
use warnings;

my @FILENAMES_ARP = <./*arp*.txt>;
my @FILENAMES_MAC = <./*mac*.txt>;
# use File::Slurp;    this way this works:  my @LINES =read_file($FILE
+ );
my %MACS;
my $FILE;


my $CLEAN_ARP_REC = './clean_arp.tmp';
open (OUTPUT, ">", $CLEAN_ARP_REC) or die "Could not open $CLEAN_ARP_R
+EC because $!"; 

for $FILE (@FILENAMES_MAC){
    print "processing $FILE...\n";
    if( open(FH, "<", $FILE) ){
        while(<FH>){
            chomp;
            next unless /^\*/; # skip if line does not start with "*",
+ for example, empty lines
            next if /static/; # skip static entries
            my @F = split(/\s+/, $_);
            my $VLAN = $F[1];
            my $MAC =$F[2];
            my $INT = $F[7];
            next unless $MAC=~/^[0-9a-f]{4}\.[0-9a-f]{4}\.[0-9a-f]{4}$
+/;
            if($MACS{$MAC}){
                warn "$MAC already defined, skipping entry at line $.\
+n";
            }else{
                $MACS{$MAC}{VLAN}=$VLAN;
                $MACS{$MAC}{INT}=$INT;
            }
        }
        close FH;
    }else{
        warn "Could not open $FILE, skipping. Errormsg=$!\n";
    }
}    

for $FILE (@FILENAMES_ARP){
    print "Processing $FILE...\n";
    if( open(FH, "<", $FILE) ){
        while(<FH>){
            chomp;
            my @F = split(/\s+/, $_);
            my $MAC = $F[3];
            my $VLAN = $F[5];
            my $IP = $F[1];
            next unless $IP; # must be defined?
            if($MACS{$MAC}){
                $MACS{$MAC}{IP}=$IP; # add this extra data
                $MACS{$MAC}{_}=1; # have to "merge"
            }
        }
        close FH;
    }else{
        warn "Could not open $FILE, skipping. Errormsg=$!\n";
    }
}

for my $MAC (sort keys %MACS){
    if( defined $MACS{$MAC}{_} ){
        print OUTPUT $MAC 
            . ", " . $MACS{$MAC}{IP}
            . ", " . $MACS{$MAC}{VLAN}
            . ", " . $MACS{$MAC}{INT}
            ."\n";
    }else{
        # normal line, do not print ?    
    }
}

close OUTPUT;
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