my @filelines = <FILE>;   #Read the entire file into an array
my @whichline;            #Declare a variable to hold the line number
for(my $i = 0; $i = $#filelines; $i++){  #Go through the entire array untill you find it or run out of lines
   if($filelines[$i] =~ m/TEXTTOMATCH/o){ #Attempt to match, use compile-once
      push(@whichline,$i); #If match, add to the list of current matches
   }
}
#Now you know what lines had TEXTTOMATCH in them