#!/usr/bin/perl -w

use strict;

my $foo = "foo";
my $bar = "bar";

my %hfoo1 = (
        "$foo$bar" => "foobar",                 # Works
        $bar.$foo  => "barfoo",                 # Works
);

my $hfoo2 = {
        "$foo$bar" => "foobar",                 # Works
        $bar.$foo  => "barfoo",                 # Works
};

my %hfoo3 = (
        map { $foo.$bar  => "foobar" } 1,       # Works
);

my %hfoo4 = (
        map { "$foo$bar" => "barfoo" } 1,       # Syntax error?
#                                    ^^^
);