I believe I saw this first in "Effective Perl Programming": 
$str =~ s/\G\s/:/g;

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The \G anchor means "anchor to where the last //g match left off". When no global match has been done yet, \G is synonymous to ^. Thus, each leading space is turned into a colon until a space is not found following the last one.
Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart

In reply to Re: substitute leading whitespace by japhy
in thread substitute leading whitespace by Anonymous Monk

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