I have used your same approach sometimes. The problem I hit is that if your program has been migrated to a production environment where the script is read-only, you cannot turn on/off the switch for debugging purposes.
That is why I prefer to add a command-line switch to turn on debugging: 
#!/usr/bin/perl -s
use warings
use strict;

[download]
use vars $d;

[download]
use vars qw/$d/;

print "Debugging message..." if $d;

[download]
Update
To run the script you would use perl myscript.pl -d

In reply to Re^3: Using the Perl Debugger (-d) by MaxKlokan
in thread Using the Perl Debugger (-d) by Melly

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