This uses the magical whitespace arg to split:

my $str = "  Perlmonks   is  the\tcoolest\n \tplace";
$str = join "_", split " ", $str;
print $str,$/;
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The magic strips leading whitespace and is greedy about the rest. This is a little broader than you asked for, but still does what you want.

After Compline,
Zaxo