$+: The text matched by the last bracket of the last successful search pattern.

I don't see any parens in there.

if ($frameDesc =~ /(\d+)$/ ) {
   print FH "FTP $1\n";
} else {
   print FH "FTP 0\n";
}
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my $num = $frameDesc =~ /(\d+)$/ ? $1 : 0;
print FH "FTP $num\n";
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