I'm not sure it would be faster, but you can use the nifty grep operator in conjunction with the keys function to achieve similar effects:

sub getIntensity {
    my $target = shift; # => $_[0], idiomatic
    my $tol = .3;
    my @matching_keys = 
        grep { abs($target-$_) <= $tol } keys %massint;
    my $intensity = 0;
    foreach (@matching_keys) {
        $intensity = $massint{$_} > $intensity ? 
            $massint{$_} : $intensity;
    }
    $intensity;
}
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First, you grab the keys within the tolerance range, and then you go through them to nab the greatest value (the ternary operator EXPR ? EXPR1 : EXPR2 returns EXPR1 if EXPR is true, and EXPR2 otherwise). You could, of course, do away with @matching_keys and run the foreach over the grep statement, but that doesn't illustrate the technique as clearly.

HTH

Philosophy can be made out of anything. Or less -- Jerry A. Fodor