To count multiple matches in a line, you can realize that m//g returns the matches in array context. That means that you can just say $counter += () = /patternmatchinghere/gi; for the counting.

I doubt if your error log is appended to your script after __END__ or __DATA__, so you should avoid the *DATA handle.

My rewrite:

my $counter = 0;
open LOG, "< $errlog" or die 'Can not open: ', $!;
$counter += () = /patternmatchinghere/gi while <LOG>;
close LOG;
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I've omitted opening $errlog to append, since I don't see what it does for you in this snippet.

Update: changed for to while to avoid slurping.

After Compline,
Zaxo