Better is $string =~ tr/ /-/; Don't use substitution when transliteration is applicable.

Why? I often hear this advice and it usually stems from the fiction that "tr/// is always faster than s///".

A better rule, IMHO, is to use the tool that fits best. In this case, both fit equally well. I personally prefer s/ /-/g because it will be recognized more widely.

If I felt that the requirement was likely to become something like "change ' ' to '-' and tab to '_'", then I might start with tr/ /-/ in expectation of changing it to something like tr/ \t/-_/ (which could be done with s/// but not so cleanly). While if I felt that the requirement was likely to become something like "change whitespace to '-'", then I'd start with s/ /-/g in expectation of changing it to something like s/\s+/-/g (which could be done with tr/// but not so cleanly).

In the very rare case where the performance difference between the two matters, which to use depends on your input. Benchmarking with one 10kB string I get:

       Rate  1tr  0tr   0s   1s
1tr 35435/s   --  -1% -27% -30%
0tr 35863/s   1%   -- -26% -29%
0s  48562/s  37%  35%   --  -4%
1s  50833/s  43%  42%   5%   --
[download]

[ Note that "0s" and "1s" are identical as are "0tr" and "1tr". I usually include such so that runs of each case are interleaved so I get an idea how much variability there is between runs vs. real differences in performance. ]

With a different 10kB string I get:

       Rate   0s   1s  0tr  1tr
0s  20623/s   --  -2% -38% -38%
1s  20993/s   2%   -- -37% -37%
0tr 33175/s  61%  58%   --  -1%
1tr 33522/s  63%  60%   1%   --
[download]

Note that in both cases, the speed difference between s/// vs. tr/// is only a few micro seconds on a 10kB string so this is extremely unlikely to matter either way for the vast majority of uses.

                - tye