ikegami, maybe I'm misunderstanding your second reason, but I don't think it is correct, because it implies that, under the /g modifier, prior substitutions affect subsequent matches, but consider the following variant of the original example: 

my $s = '   this is a  string';
( my $t = $s ) =~ s/(?<!\w)\s/X/g;
print $t, $/;
__END__
XXXthis is a Xstring

[download]
If I understood your second reason correctly, according to it the printout would have been: 
X Xthis is a Xstring

[download]
because after the first substitution the second space no longer matches the first part of the s///.

the lowliest monk

In reply to Re^2: substitute leading whitespace by tlm
in thread substitute leading whitespace by Anonymous Monk

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