my first suggestion: use strict;.

my second suggestion: use Data::Dumper;

learn them, love them, debug problems like this much easier.

if you used strict, you would see this error:

Global symbol "a1" requires explicit package name at practice/temp lin
+e 20.
Global symbol "a2" requires explicit package name at practice/temp lin
+e 28.
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from there, you'll see that you need to dereference.
print "$#a1\n"; becomes print "$#$a1\n"; and if ($a1[$BankCount] eq $arg) becomes  if ($a1->[$BankCount] eq $arg)

that said, i don't really understand what you're doing with   my $arg       = shift(@ARGV); --when i run your program, i get use of unitialized value where you try to compare against $arg. are you passing your $arg in off the command line?

here's what i get running your script with use strict;, Data::Dumper, and the array reference fixes:

$VAR1 = [
          1,
          2,
          3
        ];
$VAR1 = [
          4,
          4,
          4
        ];
2
Use of uninitialized value at practice/temp line 25.
Use of uninitialized value at practice/temp line 25.
Use of uninitialized value at practice/temp line 25.
Use of uninitialized value at practice/temp line 34.
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here's the code:

use strict;
use Data::Dumper;

  my @BankIN = (1,2,3);
  my @BankOUT = (4,4,4);
  use strict 'subs';

  arrayX(\@BankIN, \@BankOUT);

  sub arrayX{

     my($a1, $a2)=@_;
    print Dumper($a1);
    print Dumper($a2);
     my $BankCount = 0;
     my $arg       = shift(@ARGV);

         print "$#$a1\n";

         while ($BankCount <= $#$a1 )

             {
                 if ($a1->[$BankCount] eq $arg)

                 {
                    $arg = $a2->[$BankCount];
                    $BankCount = $#$a1;
                 }
                  $BankCount++;
              }

          print "$arg\n";
  }
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