how about this

proc doIt {label} {
    	global types   
    	set file [tk_getOpenFile -filetypes $types -parent .]
    	$label configure -text $file
 
	set	fo [open "$file" r]
	set	di [read $fo]
	set	dm [split $di "\r\n"]
	set	cn 0
	set	dv 1
	set	dv2 $dv
	set	pf 1
	foreach line $dm {
           if {$cn%$dv == 0} {
               set	fn "${pf}_split.csv"
               set     f [open $fn]
               incr pf
           }
           puts $f $line
           incr cn 
        }
}