Taking the tack that you may have given an oversimplified problem statement, would this do what you intended?

$num = 2;
while (<>) {
    if ( /(\A|\D)$num(\D|\Z)/ ) {
        print "match\n";
    }
}
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Here (\A|\D) matches the start of the string, or a non-digit. Likewise, (\D|\Z) matches a non-digit or the end of the string.

If you have rules for what surrounds your $num value in the input string, you may better limit your search.