G'day ISAI student,

"I could not find in the subroutine any usage of $_ in pattern matching. Thanks."

$_ is the default variable to match against (see perlrequick). So, $_ =~ /$pat/ can be written as just /$pat/. I can only suggest that you might need to be looking for constructs like:

if (/$pat/) { ...
for (m{$pat}g) { ...
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