Indeed, that's true enough -- I was trying to make a more general statement but failed to sufficiently generalize away from the specific problem of resuing old $1 etc. The more general point being to check the success of an operation before doing things that depend on the operation having succeeded. Thus, even when doing a list assignment of subexpressions from a match, one will generally want to test that the match succeeded prior to using the results:

if(my ($first, $second) = $source =~ /blah(this)blah(that)blah/){
    print "$first $second\n";
}
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