one way to do it is with a look behind assertion in a regular expression. (i.e. (?<!pattern) like the following:

while ( <DATA> ) {
 print if ( m/(?<!STOR)OMA$/ ); 
}

__DATA__
OMA
COMA
STORAMA
HISTOROMA
PUMA
MELANOMA
STOROMA
[download]

where m/(?<!STOR)OMA$/ pretty much looks for a pattern exactly as you described.

-enlil