Some feedback on your posted code:

• Always post a SSCCE
• Your hash is not correct (which would have been picked up with a SSCCE).
• Always Use strict and warnings
• Don't use $a or $b as your variable name because they have special meanings in Perl.
• DRY (you have repeated $a unnecessarily in your sample code).

Anyway, here is a very simple example of how I would go about it.

use strict;
use warnings;

# Using block lexical scope to data-hide %junksites
{
    my %junksites = (
        'bollyinside.com' => 1,
        'www.bollyinside.com' => 1,
        'worldtrademarkreview.com' => 1,
        'www.worldtrademarkreview.com' => 1,
    );

    sub KnownJunkSite {
        my $val = shift;
        return exists $junksites{$val};
    }
}

for my $v ('bollyinside.com', 'fred', 'www.worldtrademarkreview.com') 
+{
    print "$v: ", KnownJunkSite($v) ? "found\n" : "not found\n";
}
[download]

Running this little test program produces:

bollyinside.com: found
fred: not found
www.worldtrademarkreview.com: found
[download]

Update: Should you write a Procedural Module or an OO Module or just use a Hash?

In your case, if I wrote a module, I'd use OO. See also:

• Re: Procedural vs OOP modules (this is how I decide: OO or Procedural?)
• High Performance Game of Life (package Organism in "Perl Solution" section is a simple example of a Perl OO module based on a hash)

... though I'd also consider not writing a module at all, instead just using a hash/hashref directly, as analysed below in my reply to this reply.

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