I had replied earlier, but I think my post got reaped along with some other stuff as a result of a duplicate thread being started. Here again,

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

while (<DATA>)
{
   chomp;
   $_ = "$2 $1" if ( /^(.+)(\[.+)/ ); #swap 2 parts if both exist
   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]

For more study, I recommend: http://www.perlmonks.org/index.pl/Tutorials#Pattern-Matching-Regular-Expressions-and-Parsing