my %param = (mo => 1, tu =>1, ..., fr =>1);

##</code><code>##

my (%values, @unspecified);
while (my $line = <$data_in>) {
    my $day = get_day($line); # some function to get the day of week from the record
    if (exists $param{$day) {
       push @{$values{$day}}, $line;
    } else {
       push @unspecified, $line;
    }
}