# let's define a sub that will return the matched id IF there was a match, undef else
sub match_id {
   my $str = shift;

   if ($str =~ m{dp/(.\d+?)\z}) {  # we assume our id is "some character followed by digits"
       return "$1\n";  # will return id IF we had a match
   }

   return;  # undefined if there was no match
}

# assume you have these strings one in a line - or not
# and read the file to a list (one line = one list element)
# then you can get a list of matches like so:

my @ids = grep { defined $_ } map { match_id($_) } @lines_from_file;