I first tried a pack / unpack approach, that didn't work and then went to perlman:perlre and read a bit about the zero-width lookbehind operators ... And this is what I came up with :

#!/usr/bin/perl -w
use strict;

my $Test = "5.ae.20.b.1c.7";

print "05.ae.20.0b.1c.07\n";

# A feeble try with pack/unpack, that dosen't work
# print join( ".", unpack( "H2" x 6, pack( "H2" x 6, split( /\./, $Tes
+t )))), "\n";

# Now a regex which uses the zero-width lookbehind and works
# the "defined $1 ? "0$1" : "" part is ugly, I admit this ...
# but I see no other way around it ;-)
$Test =~ s/(?(?<![a-f0-9])([a-f0-9]))?\./(defined $1?"0$1":"")/eig;
print $Test, "\n";
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