use warnings;
use strict;
use Test::More;
sub exception (&) { eval { shift->(); 1 } ? undef : ($@ || die) }

my %_interp_map = ( '\\'=>'\\', '$'=>'$' ); # or use String::Unescape
sub interpolate {
	my $str = shift;
	$str =~ s{ (?<!\\) ((?:\\\\)*) \$ (?| (\w+) | \{(\w+)\} ) }
		{ $1.$ENV{$2} }xeg;
	$str =~ s{ \\ (.) }
		{ $_interp_map{$1} || die "unexpected backslash escape" }xeg;
	return $str;
}

local $ENV{USER} = "foobar";

is interpolate("USER"), "USER";
like exception { interpolate("\\USER") }, qr/unexpected backslash/i;
is interpolate("\\\\USER"), "\\USER";
is interpolate("\$USER"), "foobar";
is interpolate("\${USER}"), "foobar";
is interpolate("\\\$USER"), "\$USER";
is interpolate("\\\\\$USER"), "\\foobar";
is interpolate("\\\\\\\$USER"), "\\\$USER";
is interpolate("\\\\\\\\\$USER"), "\\\\foobar";
is interpolate("\\\\\\\\\${USER}"), "\\\\foobar";

done_testing;