A simple test:

my $sum = 0;
for( 1..10_000_000 ) { 
    $sum++ if coin_toss() eq 'heads';
}

print $sum;
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The result I got was 5,000,938, meaning heads happened 50.01% of the time. Seems pretty reasonable.

Most people's perceptions of randomness are notoriously faulty. For example, they seriously underestimate the number of times repeated numbers will appear in a sequence.

One suggestion: I think it is clearer to use int() to specify that you want a random integer:

    my $index = int rand @coin;
    return $coin[ $index ];
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As your code is written, you are using a floating point number as the array index. This works (because it is converted to integer), but it is a little unusual. You wouldn't write  $array[1.5]