knoppix@Microknoppix:~$ perl -Mstrict -wE '
> my $var1 = q{some_stuff_FOO};
> my $var2 = q{more_gubbins_BAR};
> my $var3 = q{lose_this_LOST};
> 
> my $path = join q{/},
>    q{},
>    map { my( $ext ) = m{([A-Z]+)$}; lc $ext } $var1, $var2, $var3;
> say $path;'
/foo/bar/lost
knoppix@Microknoppix:~$