In the Textfield control, try adding a ES_WANTRETURN option

AddTextfield(
...
-addstyle => 0x1000,     #ES_WANTRETURN
...
);
[download]

There may be a named option in a recent version of Win32::GUI, or maybe a symbol for ES_WANTRETURN.

If you use The GUI Loft, there is a property called WantReturn for doing this.

/J