my $ent = $mw->Entry(-textvariable    => \$value,
                     -validate        => 'key',
                     -validatecommand =>  sub { $_[0] =~ /^(?:|-|\d+|-\d+)$/ }, # <== new regex
                     -invalidcommand  => \&lam_num_error)->pack();

...

sub print
{
    print $value unless $value eq '-';
}