Now I just saw Perlbotics has posted an Inline C solution already but anyway, this is a bit simpler: 
use Inline C;
print char_freq_diff_sum("123456", "abcdefghijkl"),"\n";

__DATA__
__C__
#define abs(x) (x<0?-x:x)   /* no need for math.h */

int char_freq_diff_sum(char *s, char *t) {
    int i, sum=0, freqs[256]={0};
    for(i=0; i<strlen(s); ++i) freqs[s[i]]++;
    for(i=0; i<strlen(t); ++i) freqs[t[i]]--;
    for(i=0; i<256; ++i) sum += abs(freqs[i]);
    return sum;
}

[download]
Of course it makes the same dangerous assumptions like 8-bit character set. Unicode from UCS2 should be possible using the same principle though, a couple orders of magnitude slower but still much faster than pure Perl.

In reply to Re: The sum of absolute differences in the counts of chars in two strings. by mbethke
in thread The sum of absolute differences in the counts of chars in two strings. by BrowserUk

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