Anonymous Monk,
I wrote a tutorial on this topic. This looks like a job for grep

my @sentences = (
    'This is a sentence',
    'This is another sentence',
);
my $matches = grep /\bthis is\b/i , @sentences;
[download]

Ok, after re-reading your question several times I am not sure this answers your question. Are you asking for:

• The number of words that are the same in each sentence
• How many words are the same starting at the beginning of each sentence
• How many sentences does the phrase "This is" appear in (my first solution)
• Something else entirely

Assuming you only have two sentences and assuming you are looking for the second one:

print Start_In_Common( 'This is a sentence', 'This is another sentence
+' );
sub Start_In_Common {
    my ($first, $second) = @_;
    ($first, $second) = ($second, $first) if length $first > length $s
+econd;
    my @word = ( [split /\s+/, $first ], [split /\s+/, $second ] );
    my @fragment;
    for ( 0 .. $#{ $word[0] } ) {
        if ( $word[0][$_] eq $word[1][$_] ) {
            push @fragment, $word[1][$_];
        }
        else {
            return join ' ', @fragment, scalar @fragment;
        }
    }
}
[download]

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