This seems to work.

$s = 'the_quick_brown_fox_jumps_over_the_lazy_dog';
$s =~ s[(.)([^_]*?)(?:_|$)][\U$1\E$2 ]g;
print $s;

The Quick Brown Fox Jumps Over The Lazy Dog

$s = 'fred';
$s =~ s[(.)([^_]*?)(?:_|$)][\U$1\E$2 ]g;
print $s;

Fred
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