Perl is not bash, and has different interpolation syntax. If you want to interpolate the value of an environment variable into a string in Perl, you say $ENV{USER}, not $USER.

As to why you got a literal /a/b/c/$ENV{USER}, are you sure you used "/a/b/c/$ENV{USER}" and not '/a/b/c/$ENV{USER}'? The single quotes do not interpolate.