$debug = 1;

##</code><code>##

sub Debug {
    if ( $debug ) {
        my $msg = shift;
        print $msg, "\n";
    }
}

##</code><code>##

Debug( "Entering calculation phase." );
Debug( "Pre-calculation value of \$x: $x" );
...
Debug( "Leaving calculation phase." );
Debug( "Post-calculation value of \$x: $x" );