use warnings;
use strict;
use Data::Dumper;

my %person;
my $key;

while (<DATA>) {
    for ( split /\s+/, $_ ) {
        if (/^(ID)=(.)/) {
            $key = $2;
            $person{$key} = { lc $1 => $2 };
        }
        else {
            my ( $new_key, $value ) = split /=/, $_;
            $person{$key}{ lc $new_key } = $value;
        }
    }
}

print Dumper \%person;

__DATA__
ID=1 First=John Last=Doe AGE=42
ID=2 First=Jane Last=Doe AGE=35
ID=3 First=Jack Last=Doe AGE=17
ID=4 First=Jill Last=Doe AGE=15

##</code><code>##

        $key = $1 if /^ID=(.)/;

        my ( $new_key, $value ) = split /=/, $_;
        $person{$key}{ lc $new_key } = $value;