That is one way to take a copy of an array. With that line added to your your program, it is effectively equivalent to:

#!/usr/bin/perl -w
use strict;

my @a = (1,2,3,4);
my @p = @a;

print "before:\n";
foreach (@p) { print "element $_\n"; }

@a = ();

print "after:\n";
foreach (@p) { print "element $_\n"; }
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