$num =~ s/\G0/ /g;
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Ahh yes. I remember creating that in response to a similar question a few years ago in comp.lang.perl.misc, and then Joseph copied it for his Effective Perl Programming book, and now it's become part of the culture. Not quite as impactful as the Schwartzian Transform, but I remember getting a few "ooohs" and "aaaahs" over the simplicity of that.

-- Randal L. Schwartz, Perl hacker