One way:

c:\@Work\Perl\monks>perl -wMstrict -e
"my $data = qq{cfg_a\ncfg_b[2:0]\ncfg_c[4:0]\ncfg_d\n};
 print qq{[[$data]] \n\n};
 ;;
 open my $fh, '<', \$data or die qq{opening string by reference: $!};
 ;;
 my $identifier = qr{ [[:alpha:]] [_[:alpha:]]* }xms;
 my $bracket    = qr{ \[ \d+ : \d+ \] }xms;
 ;;
 while (my $line = <$fh>) {
   my $matched = my ($id, $br) = $line =~ m{
     \A ($identifier) ($bracket?) \Z
     }xmsg;
   die qq{bad line: '$line'} unless $matched;
   printf qq{logic%s%s\n}, map { $_ ? qq{ $_} : '' } $br, $id;
   }
"
[[cfg_a
cfg_b[2:0]
cfg_c[4:0]
cfg_d
]]

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

Update: Don't worry about the odd looking

my $data= qq{cfg_a\ncfg_b[2:0]\ncfg_c[4:0]\ncfg_d\n};
...
...
open my $fh, '<', \$data or die qq{opening string by reference: $!};
[download]

at the start. That's just a quick way to generate an example. I assume you know how to open and read a file in the more usual way. Also, please see perlre, perlretut, and perlrequick.

Update 2: Actually, the  /g modifier in the  m// above is redundant (but harmless). This
    my $matched = my ($id, $br) = $line =~ m{ ... }xms;
works just as well.