c:\test>perl -wE"open my $a, +$b;"
Name "main::b" used only once: possible typo at -e line 1.
Use of uninitialized value $b in open at -e line 1.

c:\test>perl -wE"open my $a, ''.$b;"
Name "main::b" used only once: possible typo at -e line 1.
Use of uninitialized value $b in concatenation (.) or string at -e line 1.