You should push the grepped list onto @matched, not assign. The way it is written you only match the last date. Besides, the grep is messed up.

You should also use strict and warnings; Data::Dumper is not used in your script, so nothing will be printed (and you don't get a warning).

The corrected script:

#!/usr/bin/perl
use warnings;
use strict;
use Data::Dumper;
my $driver = "<dirlist.csv";
open DRV , $driver or die "Cannot open $driver: $!";


while( <DRV> ){
    chomp;
    my ($dirname) = shift;
    checkExistingDates($dirname);
}
close DRV;

sub checkExistingDates{
    my $dirname = shift;
    my @datelist = ("20030901", "20061017", "20050406", "20070101", "2
+0080202");
    my @fileslist = ("DIR22.20060816", "DIR22.20050919", "DIR22.200610
+17", "DIR22.20060516", "DIR22.20050406");

    my @matched = ();
    foreach my $date (@datelist) {
        push (@matched, grep {/$date/} @fileslist);
    }
    print Dumper @matched;
}
[download]

But the hash solution mentioned above is much better.

Upd: btw, $dirname is read from the command line, not from the file. I suppose you meant something along the lines of my $dirname = $_ or my $dirname = (split /,/)[0].