#!/usr/bin/perl -w
use strict;
$\="\n";

## first, to BrowserUk's misunderstanding, an explanation

my $s = 'ABCD';
print substr($s,0,2)=''; # EXPECT 'CD', what's left
print $s;                # CD
$s = 'ABCD';
print substr($s,0,2,''); # EXPECT 'AB',
print $s;                # CD

# perldoc -f substr
#   An alternative to using substr() as an lvalue is to specify the
#   replacement string as the 4th argument. This allows you to
#   replace parts of the EXPR and return what was there before in
#   one operation, just as you can with splice().


my @B = 1..4;
print splice(@B,0,2,()); # expect 12
print @B;                # expect 34

# perldoc -f splice
#   Removes the elements designated by OFFSET and LENGTH from an
#   array, and replaces them with the elements of LIST, if any. In
#   list context, returns the elements removed from the array.


# now to tye's argument, saying that substr($s,0,2)= EXPR 
# doesn't return EXPR and is therefore a bug

# perldoc -f substr
#   You can use the substr() function as an lvalue, in which case
#   EXPR must itself be an lvalue.

$s = '1234';
@B=();
$B[0] = substr($s,0,2) = 'ab';
print $s;    # EXPECT ab34
print "@B";  # EXPECT ab

print "ASSIGN '' ";
$s = '1234';
@B=();
$B[0] = substr($s,0,2) = '';
print $s;    # expect 34
print "@B";  # EXPECT 34, cause '' wouldn't be an lvalue

print "ASSIGN undef ";
$s = '1234';
@B=();
$B[0] = substr($s,0,2) = undef;
print $s;    # expect 34
print "@B";  # EXPECT 34, cause undef wouldn't be an lvalue

## CONCLUSION
## if '' and undef are lvalues, then this is a feature, and not a bug

__END__
CD
CD
AB
CD
12
34
ab34
ab
ASSIGN ''
34
34
ASSIGN undef
Use of uninitialized value in scalar assignment at BrowserUk.substr.pl line 51.
34
34