use strict;
use warnings;

my ($quote_on,$output);

$_ = q/this is a "test of some regexp" blah blah
this is another "test of some regexp" foo bar/;

for (split //) {
  $quote_on = ($quote_on ? 0 : 1) if (/\"/);
  s/\s/_/g if $quote_on;
  $output .= $_;
}

print "output: $output\n";