$comm = '(xyz';
$line = '123(xyz)456';

if ($line =~ /$comm/) {     #error! same as  $line =~ /(xyz/  which has an unmatched '('
 # do stuff
}
if ($line =~ /\Q$comm/) {   #no error! same as  $line =~ /\(xyz/  -- '(' is escaped
 # do stuff
}