Please read: Markup in the Monastery, I think you need code tags to show like [2:0]

#!/usr/bin/perl
use strict;
use warnings;

while (<DATA>)
{
   chomp;
   $_ = "$2 $1" if ( /^(.+)(\[.+)/ ); #swaps 2 pieces if both found
   print "logic $_\n";
}   

=Prints
logic cfg_a
logic [2:0] cfg_b
logic [4:0] cfg_c
logic cfg_d 

=cut

__DATA__
cfg_a
cfg_b[2:0]
cfg_c[4:0]
cfg_d
[download]

http://www.perlmonks.org/?node=Tutorials#Pattern-Matching-Regular-Expressions-and-Parsing may be of interest to you.