# destructive
while( my $k = each %hash ) {
    delete $hash{ $k } unless $k =~ /\d+_\d+/;
}

# nondestructive
my %hash2;
while( my( $k, $v ) = each %hash ) {
    $hash2{ $k } = $v if $k =~ /\d+_\d+/;
}