I needed to switch debugging messages on and off dynamically on secure FTP server (pain with uploading programs - needs password), so I did not want to change source code. Instead, if file  dbgviewmode.YES exist in my developer cgi-bin directory, messages are printed. To switch them off, I'll just rename file to  dbgviewmode.NO

Routine dbg_print prints debug message (in smaller font and dark red color) together with program line and file name where it was called from.

sub is_dbgview { return -e 'dbgviewmode.yes'; }  
sub dbg_print (@) {
    my @inpar = @_;
    return 0 if not is_dbgview();
    # use caller() to figure out out procedure call stack 
    # with program names and line numbers
    my $caller = ....;
    print '<FONT COLOR=firebrick SIZE=-1>', 
          "DBG message from $caller...:,  @inpar,
          '</FONT>'; 
}
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You may want to print some comments about process status using dbg_print as well. Easy to see them if needed