use 5.14.0;

sub endOfBlock {
    my ($iface, $ref_alsHoA) = @_;
    say "interface: $iface";
    for my $k (sort keys %$ref_alsHoA) {
        say "$k: ", join ', ', @{$ref_alsHoA->{$k}};
    }
    %$ref_alsHoA = ();
}

my %alsHoA;
my $iface;
while (<DATA>) {
    if (/^###### interface (.+) ######$/) {
        endOfBlock($iface, \%alsHoA) if defined $iface;
        $iface = $1;
    }
    if (/^(Information \d+)(.+)/) {
        push @{$alsHoA{$1}}, $2;
    }
}
endOfBlock($iface, \%alsHoA) if defined $iface;

__DATA__
###### interface INFORMATION1 ######
Information 1a
Information 1b
Information 1c
Information 2a
Information 3a
Information 3b
###### interface INFORMATION2 ######
Information 1a
Information 1b
Information 1c
Information 2a
Information 3a