It was the mention of  padx in the OP title that brought  -ipadx -ipady to mind. These are pack options, but apparently  -ipadx doesn't do quite what I imagined; this

c:\@Work\Perl\monks>perl -wMstrict -le
"use Tk;
 ;;
 my $mw = tkinit;
 ;;
 my $lb = $mw->Listbox->pack(-ipadx => 10);
 $lb->insert('end', $_) for 'four score and', 'seven years ago', 'our 
+fathers';
 ;;
 MainLoop
"
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