my $debug = 2;

# This is level 3 information
if ($debug >= 3) { print DEBUG "LEVEL 3: bar\n"; }

# This is level 2 information
if ($debug >= 2) { print DEBUG "LEVEL 2: foo\n"; }

# This is level 1 information
if ($debug >= 1) { print DEBUG "LEVEL 1: moo\n"; }