18:23 >perl -wE "my $s = qq[abc\n\t  \ndef\n  \ngh]; $s =~ s/^\s*$/X/mg; say $s;"
abc
X
def
X
gh

18:24 >

##</code><code>##

18:24 >perl -wE "my $s = qq[abc\n\t  \nd\tef\n  \ngh]; $s =~ s/[ \t]+/X/mg; say $s;"
abc
X
dXef
X
gh

18:39 >