As wise Corion you are not checking the right thing:

you start the script in the directory /one then you open /one/two/three reading it. If you say cd four you'll go in /one/four not in /one/two/three/four

Take a read of Cwd

Tested:

#!usr/bin/perl

# ADD THESE TWO LINES!
use strict;
use warnings;

# so you are forced to declare variables with my
my $path='/home/something/something/something';

opendir(DIR,$path) or die"couldnt open the directory\n";

# parens not needed
# UPDATE: assignement to $_ is needed with older perls: nowadays while
+(readdir DIR) will suffice
# as choroba spotted it is since 5.12 that readdir automatically feeds
+ $_
# doc says: As of Perl 5.12 you can use a bare readdir in a while loop
+, which will set $_ on every iteration.

while($_ = readdir DIR)
{
  # this is wrong! 
  # if(-d BUS)
  if ($_ eq 'BUS' and -d $path.'/'.$_) #corrected by Corion
   {
      # append BUS to the path
      # infact you are not changing directory

      my $buspath = $path.'/'.$_;
      opendir(VEH,  $buspath) or die"couldnt open the directory  [$bus
+path]\n";
      while($_ = readdir VEH)
       {
         print "$_\n" if -f $buspath.'/'.$_;
       }
   }
}
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