This appeared to break the exact stipulations of the OP, but on the other hand... at least the input and output rules are obeyed by this:

$string="a_sample_string";
#...
my $disposable = $string; # put it in a disposable variable
$disposable =~ s/_/ /g; # substitution on that instead
$disposable =~ /^(.*)$/; # then put it in $1
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