This sounds like it might at least be related to token parsing, in which case Parse::Token might work well for you. Otherwise, it's essentially a simple switch.

my @array;
my $start_regex = qr/^\d{3}:/; # a line beginning with, e.g. 001:
my $end_regex   = qr/^:::$/;   # a line with only ':::'
while (<DATA>) { # replace with your FH
    if ( @array and $_ =~ $end_regex ) {
        # reached an 'end'.  I assume you don't want to keep the end '
+tag'
        do_something_to(@array);
        undef @array;
    }
    elsif ( @array ) {
        # filling array
        push @array, $_;
    }
    elsif ( $_ =~ $start_regex and !eof DATA) {
        # begin filling up the array!
        push @array, $_;  # I assume you want to keep the start 'tag'
    }
    else {
        # we haven't gotten a 'start',
    }
} #_ while{}

# deal with the possibility that we 'started' but didn't 'end'
if (@array) { warn 'Start with out end at EOF!' }
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