Convert your dates to days since your membership started. Date::Manip is excellent at parsing dates in pretty much any known format (though the docs are little incomprehensible, its worth the effort persevering with them). Its function ParseDate() and Date_Calc() will do all the work for you.

The basic idea is to build up a string one char per day, '1' when they were a member and '0' when not. Then testing for membership on a particular day involves just calculating the offset for the day in question an using substr to test if there's a '1' there. For a range of days, calculate the offset and length, substr the string and use a simple m// to see if there is a '1' anywhere in the substring. That's is really.

If the length of the strings times the number of members, eg. 10years * 356 days * 4500 members = approx 16MB is considered too much then you could trade substr in for vec and use bits instead of bytes. A little more complex, but 2MB only.

I've heard that Date::Calc is almost as good at parsing strnge date formats as Date::Manip and runs much faster because its in C. I haven't had occasion t ry it.

A bit of sample code to demonstrate the idea. Kinda of works but has bugs. Might get you started though with the date stuff.

#! perl -slw
use strict;
use Data::Dumper;
use Date::Manip;

my $days_since_open = 365*3; # 3 years
my $base_date = ParseDate( '1st January 2000' );

my %members;
while(<DATA>) {
    my @stuff = split/\|/;
    my $member = shift@stuff;
    $members{$member}= '0' x ($days_since_open);
#    print $member;
    while (@stuff > 1) {
        my $date1 = ParseDate( shift@stuff );
        my $start = Delta_Format( DateCalc( $base_date, $date1), 0, '%
+dh' );
        my $date2 = ParseDate( shift@stuff );
        my $length = Delta_Format( DateCalc($date1, $date2), 0, '%dh')
+;
        substr($members{$member}, $start, $length) = '1' x $length;
    }
    if (@stuff) {
        my $last = ParseDate( shift@stuff );
        my $last_offset = DateCalc($base_date, $last );
        my $current = Delta_Format( $last_offset, 0, '%dh');
        substr($members{$member}, $current) = '1' x ($days_since_open 
+- $current);
    }
}

my $start = do{{
    print 'Start date?: '; warn "Bad date, try again", redo
        unless ParseDate(scalar <STDIN>);
}};
my $end = do{ print 'End date?: ';ParseDate(scalar <STDIN>); };
#print "$start, $end";

my $start_days = Delta_Format( DateCalc( $base_date, $start ), 0, '%dh
+' );
my $end_days = Delta_Format( DateCalc( $base_date, $end ), 0, '%dh' ) 
+if $end;
#print "$start_days, $end_days";

for my $member (keys %members) {
    print $member, ' was a member ',
        $end ? 'between ' : 'on ',
        $start,
        $end ? ' and '.$end : '',
        if substr($members{$member}, $start_days, $end_days||1) =~ m[1
+];
}

=pod Output

C:\test>232212
Start date?:
1st April 2000
End date?:
^Z
Use of uninitialized value in string eq at e:/Perl/site/lib/Date/Manip
+.pm line 4155, <STDIN> line 1.
Fred was a member on 2000040100:00:00
Barney was a member on 2000040100:00:00
Wilma was a member on 2000040100:00:00

C:\test>232212
Start date?:
1st may 2000
End date?:
1st december 2002
Fred was a member between 2000050100:00:00 and 2002120100:00:00
Barney was a member between 2000050100:00:00 and 2002120100:00:00
Betty was a member between 2000050100:00:00 and 2002120100:00:00
Wilma was a member between 2000050100:00:00 and 2002120100:00:00

C:\test>232212
Start date?:
1st may 2000
End date?:
1st december 2001
Fred was a member between 2000050100:00:00 and 2001120100:00:00
Barney was a member between 2000050100:00:00 and 2001120100:00:00
Wilma was a member between 2000050100:00:00 and 2001120100:00:00

C:\test>

=cut

__DATA__
Fred| 2000-march 31 | 2000/april/7 | 19/jan/2002 | 24th february 2002 
+| 16sep2002
Barney| 15 february 2000 | Feb 15th 2002 | 1 dec 2002
Wilma| 01apr2000| 2002/19sep
Betty| 01DEC2002
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