You must make sure that your do blocks always evaluate to an intended result. In this case, you have to add an else { undef } clause.

Indeed.

In your case, the value of $foo is assigned to $bar if $foo is false.

Perl is a bit wierd about the return value of a loop of which the body never runs. This code:

#!/usr/bin/perl -w

my $true = "foo";
my $false = "0";
my $dc = "bar";

warn do { $dc if $false };
warn do { $dc unless $true };
warn do { $dc while $false };
warn do { $dc until $true };
warn do { if ($false) { $dc } };
warn do { unless ($true) { $dc } };
warn do { while ($false) { $dc } };
warn do { until ($true) { $dc } };
warn do { $dc if "0" };
warn do { $dc unless "foo" };
warn do { $dc while "0" };
warn do { $dc until "foo" };
warn do { if ("0") { $dc } };
warn do { unless ("foo") { $dc } };
warn do { while ("0") { $dc } };
warn do { until ("foo") { $dc } };


__END__
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prints:

Useless use of private variable in void context at a line 9.
Useless use of private variable in void context at a line 10.
Useless use of private variable in void context at a line 13.
Useless use of private variable in void context at a line 14.
Useless use of private variable in void context at a line 21.
Useless use of private variable in void context at a line 22.
0 at a line 7.
foo at a line 8.
0 at a line 9.
foo at a line 10.
0 at a line 11.
foo at a line 12.
0 at a line 13.
foo at a line 14.
0 at a line 15.
foo at a line 16.
0 at a line 17.
Warning: something's wrong at a line 18.
0 at a line 19.
Warning: something's wrong at a line 20.
Warning: something's wrong at a line 21.
Warning: something's wrong at a line 22.
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