TIMTOWTDI, you don't really have to use recursion here, I have written a non-recursive demo that does what you described...

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

my $card = load_vcard();
print Dumper($card);

sub load_vcard
{
    my $n = 0;
    my @var = ();
    foreach (<DATA>) {
        chomp;
        $n++, next if /^BEGIN:VCARD/;
        $n-- ? last : next if /^END:VCARD/;
        next if /^AGENT/;
        push @var, /FN:(\w+)/;
    }

    my %card;
    eval '$card' . '{AGENT}' x $_ . '{FN} = "' . $var[$_] . '"'
        for (0..$#var);

    return \%card;
}

__DATA__
BEGIN:VCARD
FN:Fee
AGENT:
BEGIN:VCARD
FN:Fie
AGENT:
BEGIN:VCARD
FN:Foe
AGENT:
BEGIN:VCARD
FN:Fum
END:VCARD
END:VCARD
END:VCARD
END:VCARD
[download]

And the output is -

$VAR1 = {
          'AGENT' => {
                       'AGENT' => {
                                    'AGENT' => {
                                                 'FN' => 'Fum'
                                               },
                                    'FN' => 'Foe'
                                  },
                       'FN' => 'Fie'
                     },
          'FN' => 'Fee'
        };
[download]