sub popstr   { substr $_[0], -1, 1, '' } # or chop
sub shiftstr { substr $_[0],  0, 1, '' }
sub pushstr    { length($_[0] .= join '', @_[1..$#_]) }
sub unshiftstr { length($_[0]  = join('', @_[1..$#_]) . $_[0]) }

my $string = 'testing';
my $foo = shiftstr $string; # esting
my $bar = popstr $string;   # estin
pushstr $string, $foo;      # estint
unshiftstr $string, $bar;   # gestint
print "$string\n";