I think you need to extend your match to include, either the start/end of the line or a single space, after radio. This will capture cases where radio is in the middle of a phrase, or if it is at the start or end of your sentence.

Use |s enclosed in parens to specify alternatives.

Try something like below:

#!/usr/bin/perl -w


my @sentence_list = (
            'radiohead',
            'turn off your radio',
            'jill has a radio that is black',
            'the stereo is broken',
            'radio for happiness',
           );

foreach $sentence (@sentence_list){
    if ($sentence =~ /(^|\W)radio(\W|$)/){
        print "Found\n";
    } else {
        print "Not found\n";
    }
}
[download]

This gives results:

Not found
Found
Found
Not found
Found
[download]