for '0001' .. '1999'

##</code><code>##

for '0001' .. '1999'

##</code><code>##

for '0001' .. '\'1999\''

##</code><code>##

use warnings;

for('0001' .. 1999) {print "\r$_"}
print "\n";

for('0001' .. '1999') {print "\r$_"}
print "\n";

for('0001' .. '\'1999\'') {print "\r$_"}

__END__
Outputs:
1999
1999
999999