If you are just beginning to learn regexes, the negative look-behind examples given in the other posts here may feel a little challenging. Here's an approach that uses more basic regex functionality.

#!/usr/bin/perl -w
use strict;

my @words = qw(COMA STORAMA HISTOROMA PUMA MELANOMA STOROMA);

for my $str ( @words ) {
   if ($str =~ /OMA$/ and $str !~ /STOROMA$/ ) {
      print "$str\n";
   }
}
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"Perl is a mess and that's good because the
problem space is also a mess." - Larry Wall