my $f = $top->Frame()->pack(-expand => 1);
my $e = $f->Entry(-width => 50,)->pack(side => 'left', -expand => 1, -fill => 'x');
my $such_b = $f->Button(-text => 'Suchen', -default => 'active', -command => \&suchen,)->pack(-side => 'left');

$top->bind ('<Return>',    sub{$such_b-> invoke()}); 

sub suchen {
    my $such = $e->get();
    ...
}