Rather than just setting it to 1, it may be more flexible to use $debug_switch++ instead; that way you could increase the debug level with every -d on the command-line, letting you do things like...

use constant VAGUE    => 0;
use constant DETAILED => 1;
use constant PEDANTIC => 2;

if ( @x ) {
    warn( "\@x is set\n"            ) if DEBUG > VAGUE;
    warn( scalar(@x)," elements\n"  ) if DEBUG > DETAILED;

    {
        local $" = "\n  ";
        warn( "They are...\n  @x\n" ) if DEBUG > PEDANTIC;
    }
}
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Lame example, but hopefully you get the point... :)

    --k.