Welcome to the monastery,
Dervish!
As suggested, you should give a minimal piece of code that will compile and run and produce the issue you're having. In case you're not aware (though you've likely guessed), the message you're seeing is a non-fatal warning, which means you're invoking the script with -w or putting
use warnings; at the top. This is usually (always?) a good thing to do.
What this particular warning is telling you is that you're attempting to use a regular expression against an undefined variable.
Something like the following, for example, would produce a similar warning to what you're seeing.
use warnings;## good line to have. use strict; is also prudent
$x=undef; ## I explicitly set it to undef here]
## even if I commented this, though, I'd have
## the same warning
($s)=$x=~m/regex/; ## $x is undefined, and produces warning
print $s; ## presumably, I wanted $s for something
You should definitely look at the line number suggested by the warning.