my $fps = 'filepathseparator';
m!
$fps             # your file path separator
\Q$\E            # quoted scalar sigil (shell variable expansion)
\{?              # start shelling out for a return value ?
([^\W]+)         # your environment variable, not containing,
                 # one or more non-word characters
\}?              # end shelling out for a return value ?
!x