Remove the join statement, then I think this does what you're after:

my %data;
my $device;

for (@check){

    next if ! /\d{1,2}-\d{1,2}/;

    chomp;
    s/\s+//g;

    if (/EQUIP-OME/){
        $device = $_;
        next;
    }
    push @{$data{$device}}, $_;
}

for my $device (keys %data){
    for my $interface (@{$data{$device}}){
        print "$device,$interface\n";
    }
}
[download]

Output:

EQUIP-OME815-08-2615:19:27,"ETH10G-1-13-2,ETH10G:OPT-OCH,-3.20,COMPL,N
+END,TRMT,1-DAY,08-25,00-00,1,,,,"
EQUIP-OME815-08-2615:19:27,"ETH10G-1-13-2,ETH10G:OPRN-OCH,-17,COMPL,NE
+ND,RCV,1-DAY,08-25,00-00,1,,,,"
115-EQUIP-OME515-08-2615:19:26,"OC12-1-4-7,OC12:OPRN-OCH,-79,COMPL,NEN
+D,RCV,1-DAY,08-25,00-00,1,,,,"
115-EQUIP-OME515-08-2615:19:26,"OC12-1-4-7,OC12:OPTN-OCH,0,ADJ,NEND,TR
+MT,1-DAY,08-25,00-00,1,,,,"
115-EQUIP-OME515-08-2615:19:26,"OC12-1-4-8,OC12:OPR-OCH,-14.04,COMPL,N
+END,RCV,1-DAY,08-25,00-00,1,,,,"
EQUIP-OME115-08-2615:19:13,"OC192-1-1-2,OC192:OPR-OCH,-10.20,COMPL,NEN
+D,RCV,1-DAY,08-25,00-00,1,,,,"
EQUIP-OME115-08-2615:19:13,"OC192-1-1-2,OC192:OPT-OCH,-3.50,COMPL,NEND
+,TRMT,1-DAY,08-25,00-00,1,,,,"
123-EQUIP-OME215-08-2615:19:25,"OC12-1-3-7,OC12:OPRN-OCH,-89,COMPL,NEN
+D,RCV,1-DAY,08-25,00-00,1,,,,"
123-EQUIP-OME215-08-2615:19:25,"OC12-1-3-7,OC12:OPTN-OCH,0,ADJ,NEND,TR
+MT,1-DAY,08-25,00-00,1,,,,"
[download]

-stevieb