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

my %result = (key1 => undef, key2 => undef, key3 => undef);
{       # So that the effect of local is limited to that block
        local $/ = 'ENDKEY';
        while( my $line = <DATA>) # <FH> in your case
        {
                next unless $line =~ m<HIT \s+ (key\d+)\b >x;
                my $key = $1;
                next unless exists $result{$key};
                $result{$key} = { attrib1 => 'null', attribmsg => 'null' };
                while ($line =~ m<\s+ (attrib1 | attribmsg) :.*? = \s+ (\w+) >gx)
                {
                        $result{$key}{$1} = $2;
                }
        }
}

print Dumper \%result;

__DATA__
HIT key1 so start to process
    attrib1:  base = FULL
        attribmsg: middle = MEDIUM
ENDKEY

HIT key2 so start to process
    attrib1:  base = FULL
ENDKEY

HIT key9 so start to process
    attrib1:  base = FULL
ENDKEY