Since your dates appear to be in a nice comparable format, you can just do ordinary string comparisons on them to find out which is oldest. So ... something like this should work fine:

#!/usr/bin/perl

use strict; use warnings;

my (%oldest_date, %oldest_entry);
while (<DATA>)
{
    my ($item,$group,$date) = split;
    if (!exists $oldest_date{$group} || $date lt $oldest_date{$group})
+ {
        $oldest_date{$group} = $date;
        $oldest_entry{$group} = $_;
    }
}

for my $g  (keys %oldest_entry) {
    print $oldest_entry{$g};
}


__DATA__
34     gr1    2003-03-02
12     gr1    1990-03-14
39     gr3    2002-04-11
66     gr4    2006-03-16
32     gr3    1998-02-13
90     gr1    2004-06-15
55     gr4    1999-06-15
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