I tried alot but couldnt able to do that please help me to do that

What you didn't try to do is describing your code to yourself; I'll do that for you:

You create VEHICLES and change into this empty directory. Fine. Inside this directory you create a directory and change into this empty dirctory.

Then, inside this subdirectory, you try to chdir into the directory VEHICLES (which is what the variable $dir holds). This subdirectory doesn't exist in VEHICLES/CARS. Bummer. This part of your code silently fails. Your current working directory stays the same (VEHICLES/CARS).

You iterate to the next element in @files.
You create the next directory, BIKES, and change directory into that. Again, you try to chdir into VEHICLES which doesn't exist.

You end up with

VEHICLES/CARS/BIKES
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which is your current working directory after the loop ends.

First thing you should do is checking all your operations for success:

$dir = "VEHICLES";
@files = ('CARS', 'BIKES');
mkdir ($dir) or die "can't mkdir $dir: $!";
chdir ($dir) or die "can't chdir to $dir: '$!'";
foreach $file(@files)
{

  mkdir $file or die "(loop) can't mkdir $file: '$!'";
  chdir $file or die "(loop) can't chdir to $file: '$!'";
  chdir $dir or die "(loop) can't chdir to $dir: '$!'";
}
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Output:

(loop) can't chdir to VEHICLES: 'No such file or directory' at try.pl 
+line 10.
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Aha! there's no such file or directory inside the loop, when you try to chdir to VEHICLES. You realize the two chdir statements inside the loop are bogus and eliminate them:

$dir = "VEHICLES";
@files = ('CARS', 'BIKES');
mkdir ($dir) or die "can't mkdir $dir: $!";
chdir ($dir) or die "can't chdir to $dir: '$!'";
foreach $file(@files)
{

  mkdir $file or die "(loop) can't mkdir $file: '$!'";
}
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Then you run your file again. Output now:

can't mkdir VEHICLES: File exists at try.pl line 3.
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So, you should check for existence before making a directory. See -X for the various file testing operators.

$dir = "VEHICLES";
@files = ('CARS', 'BIKES');
if(not -d $dir)
{
    mkdir ($dir) or die "can't mkdir $dir: $!";
}
chdir ($dir) or die "can't chdir to $dir: '$!'";
foreach $file(@files)
{
  if (not -d $file)
  {
      mkdir $file or die "(loop) can't mkdir $file: '$!'";
  }
}
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Now your code runs and produces the expected structure:

VEHICLES
VEHICLES/BIKES
VEHICLES/CARS
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See chdir, mkdir, not, -X, or, die for more information about the added bits.