unless ($$buffer =~ m/^H=(?>\d+);/) {
    confess "$rec_num was missing the header prefix m/^H=\\d+;/: $$buffer";
}

my $header_length = substr $$buffer, 2, $+[0]-3;

# vs

unless ($$buffer =~ m/^H=((?>\d+));/) {
    confess "$rec_num was missing the header prefix m/^H=(\\d+);/: $$buffer";
}

my $header_length = $1;