substr($_, 4, 0, '.');          # at least 4 characters long
substr($_, 4, 0) = '.';         # at least 4 characters long
$_ =~ s/^(.{4})/$1./;
$_ =~ s/(?<=^.{4})/./;
pos($_)=4; $_ =~ s/\G/./;       # at least 4 characters long
$_ .= '.' . chop($_);           # exactly 5 chars long
$_ /= 10;                       # a 5 digit number without leading 0s
$_ = sprintf('%06.1f', $_/10);  # a 5 digit number