$ cat /tmp/p
#!/usr/bin/perl

@a = qw(1 2 3);
@b = qw(a b c d);

push @{ $x ? @a : @b }, 'foo';

print "a: @a\n";
print "b: @b\n";
print "3: @3\n";
print "4: @4\n";
__END__

$ ./perl /tmp/p
a: 1 2 3
b: a b c d
3: 
4: foo