I tend to only use the first option in the following situation
$debug && print "Some debugging output.\n";
Works for me because I see the "$debug" and mentally
disregard that line from the regular functionality of the
code.
print "Some debugging output.\n" if $debug;
...doesn't stand out as much as debugging because I tend
to scan code down the left side.
if ($debug) {
print "Some debugging output.\n";
}
...that way just eats up way too much room when there's a
lot of debugging output.
And of course these are all for smaller scripts where a more
complete debugging output solution isn't needed.