Something like:

my %filename;
while (<DATA>) {
    warn "No match for $_" unless m#^(\w+?\d+)#;
    $filename{$1}++;
}
print "$_\n" foreach sort keys %filename;
__DATA__
smith_13_503_de7.p
smith_13_502_de7.v
jones_104_503_de7.p
jones_104_502_de7.v
[download]

Please note the use of +? to prevent \w from being greedy and eating everything until the period.

Liz