The || operator has higher precedence than the = operator so you need to either enclose the assignment in parentheses or use the lower precedence or operator.
No cigar for you :)

C:\>perl -MO=Deparse,-p -e" my $foo = shift || die 666;"
(my $foo = (shift(@ARGV) || die(666)));
-e syntax OK

C:\>perl -MO=Deparse,-p -e" my $foo = shift or die 666;"
((my $foo = shift(@ARGV)) or die(666));
-e syntax OK

C:\>perl -e" my $foo = shift || die 666;"
666 at -e line 1.

C:\>perl -e" my $foo = shift or die 666;"
666 at -e line 1.

C:\>
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Perl Idioms Explained - && and || "Short Circuit" operators
To || or not to or and why.
|| die or or die :)
"or" or "||" problems
or vs || precedence issue