I'm assuming that q->param{'at0'} is a reference to an array.

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

# array : assume address 123

    my @a = qw(one two three);
    
# reference to array in address 123  
# (ARRAY(0x123)    

    my $aa = \@a;           
    
# referencec to anonymous array containing 
# one element, the reference to array @a, 
# (ARRAY(0x123))
# assume $bb = ARRAY(0x789)

    my $bb = [$aa];             

# @{$aa} returns the array at address 123
# - which is just array @a,
# the \@{$aa} returns a reference to the
# array at address 123.
# - so $cc = ARRAY(0x123)

    my $cc = \@{$aa};

# results

    print "array a <@a>\n";
    print "reference to array a <$aa>\n";
    print "reference to anon array containing reference to array <$bb>
+\n";
    print "contents of array referred to by \$bb <@$bb>\n";
    print "reference to the array referred to by \$aa <$cc>\n";
    print "contents of array referred to by \$cc, or array a <@$cc>\n"
+;
[download]

Result

array a <one two three>
reference to array a <ARRAY(0x148bac)>
reference to anon array containing reference to array <ARRAY(0x138a6c)
+>
contents of array referred to by $bb <ARRAY(0x148bac)>
reference to the array referred to by $aa <ARRAY(0x148bac)>
contents of array referred to by $cc, or array a <one two three>
[download]

UPDATE Changed the word 'pointer' to 'reference', as per revdiablo's comment.