Here's what I've come up with so far (using some of the improvements suggested and benchmarking using GrandFather's example):

my $hdr_search = qr{^\-{4}\s\d};

sub useIndex {
    my ($ip1, $ip2, $lines) = @_;

    for my $line (@$lines) {
        unless (($line =~ $hdr_search) || ($header)) {
            next;
        }
        unless ($line =~ $hdr_search) {
            print $line;  ## Print matching packet body
            next;
        }
        if (0 > index ($line, $ip1) || (0 > index ($line, $ip2))) {
            $header = 0;
            next;
        } else {
            print "\n$line"; ## Print matching packet header
            $header = 1;
        }
    }
}
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Using the index function along with getting rid of IF cases early has definitely sped things up considerably. Thoughts?