Anything's possible with a little help from OLE (and an hour to kill reading Excel docs;-)

#!/usr/bin/perl

use strict;
use warnings;
use Win32::OLE;
use Win32::OLE::Const "Microsoft Excel";
use Win32::OLE::Const "Microsoft Visual Basic for Applications Extensi
+bility";

my $xls = Win32::OLE->new('Excel.Application');
$xls->{Visible} = -1;
my $wb = $xls->Workbooks->Add();
my $mod = $wb->VBProject->VBComponents->Add(vbextFileTypeModule);
$mod->CodeModule->AddFromString ( <<MODTEXT);
Sub Message
    dim s as string
    s = "Hello, World"
    MsgBox s
End Sub
MODTEXT

my $sheet = $xls->Sheets('Sheet1');
my $button = $sheet->Buttons->Add(71.25, 18.75, 88.5, 57);
$button->{OnAction} = 'Message';
[download]

Notes:

• Error checking is left as an exercise.
• I tested this with Excel 97.
• VBProject objects are horribly documented in the Excel helpfiles. I ended up using the object browser from ActiveState to put it all together.