open (EXT, "LDB_EXTID.lst") or die "something";
my @uids = <EXT>; # read in each line as a member of the array
close EXT;

# here's the key line !
chomp @uids; # remove any trailing newlines from every member of @uids

foreach my $uid (@uids) {
    my $output = `ldbtest.pl -e $uid -t 123`;
    last if ++$counter > 10;
}
# stuff you'd do after the loop, if anything