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

##</code><code>##

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 $second;
    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;
        }
    }
}