You need to quote the meta characters in the regex with \Q and \E. Otherwise you need to escape the meta-characters (such as [ and .) with backslash (\).

Here's an example:

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

my $grp = qr/\[\d+-\d+\]/;

for (<DATA>) {
    print if /\d+\.\d+\.$grp\.(\d+|$grp)/;
}


__DATA__
123.145.[146-149].2
135.168.[10-115].[0-125]
135.168.12.[0-125]
[download]

Update:
Sorry I misread the question. If you are after a single regex to do range checking, you can use the match time interpolation technique.

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

my $grp = qr/^123\.145\.(\d+)\.(\d+)$(??{ $1 >= 146 && $1 <= 148 ? '' 
+: 1})/;

for (<DATA>) {
    print if /$grp/;
}


__DATA__
123.145.130.3
123.145.146.3
123.145.147.3
123.145.148.3
123.145.149.3
[download]