The hint to perlfaq4 is excellent.

Use a hash and grep() and you are fine. No need for delete() or splice() here.

#! /usr/bin/perl
use strict;
use warnings;

my @arcb = ( 450, 625, 720, 645 );
my @arca = ( 625, 645 );

### see perlfaq4 - How can I remove duplicate elements from a list or 
+array?
my %unwanted;                           ### or %seen as in the faq4
$unwanted{$_}++ for @arca;

@arcb = grep { !$unwanted{$_}  }  @arcb;

print "@arcb\n";                                                      
+        

__END__
[download]