$ perl -Mstrict -Mwarnings -E '
    my @data = (
        q{1:1 word1},
        q{1:1 word2},
        q{1:1 word3},
        q{1:1 word4},
        q{1:2 word1},
        q{1:2 word2},
        q{1:2 word3},
        q{1:3 word1},
        q{1:3 word2},
        q{1:3 word3}
    );

    my $old_head = q{};

    for (@data) {
        my $new_head = (split)[0];

        if ($new_head ne $old_head) {
            say "> $new_head";
            $old_head = $new_head;
        }

        say;
    }
'
> 1:1
1:1 word1
1:1 word2
1:1 word3
1:1 word4
> 1:2
1:2 word1
1:2 word2
1:2 word3
> 1:3
1:3 word1
1:3 word2
1:3 word3