$ perl -wMstrict -le 'my $fn="x"; print "$fn.y"'
x.y
$ perl -wMstrict -le 'my $fn="x"; print "$fn_y"'
Global symbol "$fn_y" requires explicit package name (did you forget to declare "my $fn_y"?) at -e line 1.
Execution of -e aborted due to compilation errors.
$ perl -w        -le 'my $fn="x"; print "$fn_y"'
Name "main::fn_y" used only once: possible typo at -e line 1.
Use of uninitialized value $fn_y in string at -e line 1.
$ perl -wMstrict -le 'my $fn="x"; print "${fn}_y"'
x_y