What you want is the difference between the two arrays. That can be computed like this.

#! perl -slw
use strict;

my @before_files = map{ chr 65+rand 26 } 1.. 4;

my @after_files  = ( @before_files,  map{ chr 65+rand 26 } 1.. 6 );

my %seen; @seen{ @before_files } = ();

my @only_new_files = grep{ not exists $seen{ $_ } } @after_files;

print "@before_files\n@after_files\n@only_new_files";

__END__
P:\test>junk2
Y B Y A
Y B Y A P E F N L U
P E F N L U
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