perldebug. However you may find it easier to add code in the form:

my $DEBUG = 0;   # set to > 0 for debugging, the bigger the value the 
+more verbose

# in the code wherever you think it useful
$DEBUG && DEBUG( 2, "some message" );


sub DEBUG {
    my ( $debug_level, $message ) = @_;
    # see if we want this message a current debug level
  return if $debug_level > $DEBUG;
    # we can get info on where this info comes from from caller()
    my ( $package, $file, $line ) = caller();
    # print a standard format message, say line num and message
    warn "$line\t\$message\n";
}
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This is one of many ways to incorporate debugging. Just call the DEBUG function with an int first argument and a message. Set the debug level and get more verbose output. The $DEBUG && DEBUG( ... ) suntax is to avoid the sub call if we are not debugging.