use Win32::GuiTest qw/
   FindWindowLike 
   SetForegroundWindow 
   GetDesktopWindow 
   IsWindowVisible
/;

my @handles =   grep {IsWindowVisible($_) } 
                FindWindowLike(GetDesktopWindow(),
                               qr/^Microsoft Excel/,
                               undef,
                               undef);

die "Too many windows\n" if (@handles > 1);
die "Not enough windows\n" if (@handles < 1);

my $excel = shift @handles;

SetForegroundWindow($excel);