---

use strict;
use warnings;

my %replace = qw(
    COM  SEC

    MOC  COM
    SEC  COM
    CO   MOC
);
my $searchpat = join '|', sort { length $b <=> length $a } keys %repla
+ce;
$searchpat = qr/$searchpat/; # optional, dunno if it speeds up things

my $example = "FOO CO COM BAR MOC SEC";
$example =~ s/($searchpat)/$replace{$1}/g;
print "$example\n";
[download]

Produces the output:

FOO MOC SEC BAR COM COM
[download]

As you can see, it also handles circularity without problem; COM <-> SEC.

Note that I'm sorting the keys on length to make sure longer patterns match first (otherwise "COM" would become "SECM").

---

If the search-string needs to occur as a word by itself, use \b, like:

my $example = "MOC.COMPUTER.COM.FOO / SEC.SECT / CO";
$example =~ s/\b($searchpat)\b/$replace{$1}/g;
print "$example\n";
[download]

which prints:

COM.COMPUTER.SEC.FOO / COM.SECT / MOC
[download]

Note that in this case sorting on the length of the key is not necessary, so you can simply do my $searchpat = join '|', keys %replace;

---

Final note: this code assumes the keys don't contain any odd chars like regex metachars. Since your example shows all-uppercase abbreviations, so apparently this isn't a problem. If it is, then you'll need to look into using quotemeta.